Why do boxers of today have so few fights compared to the olden day fighters?

Early on in a boxers career these days he might have a fight every two months. The top fighters of today, however, have around 2-3 fights per year and if they're lucky may finish with 50 fights. If you look at the records of the fighters of the past centuries they had around 150-200, some even surping the 300 mark. And in those days there was no round limit. What is the reason for the difference today? Is it down to the boxers physical condition? Is it down to money? Is it down to the IBA? Surely if they fought like every two months or so at least then there going to earn more money, the primary reason they are in the game. It's not like some of these even go 12 rounds. Some of the top guys go in and KO the opponent in two rounds or so. Yet he still waits 4-6 months until his next fight. Another question I have is when did it get to the way it is today? Was there a particular turning point?